Pterodaustro isometric growth series

Tradtional paleontologists think pterosaur babies had a cute short rostrum that became longer with maturity and a large orbit that became smaller with maturity (Fig. 1). This is a growth pattern seen in the more familiar birds, crocs and mammals.

Figure 1. Pterodaustro embryo as falsely imagined in Witton 2013. The actual embryo had a small cranium, small eyes and a very long rostrum.

Unfortunately
these paleontologists ignore the fossil evidence (Figs 2, 3). These are the data deniers. They see things their own way, no matter what the evidence is. The data from several pterosaur growth series indicates that hatchlings had adult proportions in the skull and post-crania. We’ve seen that earlier with Zhejiangopterus (Fig. 2), Tapejara, Pteranodon, Rhamphorhynchus and others. Still traditional paleontologists ignore this evidence as they continue to insist that small short rostrum pterosaurs are babies of larger long rostrum pterosaurs.

Figure 2 Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.

As readers know,
several pterosaur clades went through a phase of phylogenetic miniaturization, then these small pterosaurs became ancestors for larger clades. Pterosaurs are lepidosaurs and they grow like lepidosaurs do, not like archosaurs do.

Today we’ll look at
the growth series of Pterodaustro (Fig. 1), previously known to yours truly only from adults and embryos. Today we can fill the gaps with some juveniles.

This blog post is meant to help traditional paleontologists get out of their funk.

A recent paper
on the braincase of odd South American Early Cretaceous pterosaur Pterodaustro (Codorniú et al. 2015) pictured three relatively complete skulls from a nesting site (Fig. 1). I scaled the images according to the scale bars then added other available specimens.

Figure 1. Pterodaustro skulls demonstrating an isometric growth series. One juvenile is scaled to the adult length. One adult is scaled to the embryo skull length. There is no short rostrum and large orbit in the younger specimens. If you can see differences in juvenile skulls vs. adult skulls, please let me know. All these specimens come from the same bone bed.

You can’t tell which skulls are adults or juveniles
without scale bars and/or comparable specimens. As we established earlier, embryos are generally one-eighth (12.5%) the size of the adult. Pterodaustro follows this pattern precisely.  We have adults and 1/8 size embryos and several juveniles of intermediate size.

No DGS was employed in this study.

If you know any traditional paleontologists, 
remind them that the data indicates that pterosaurs matured isometrically, like other  lepidosaurs. Those small, short rostrum specimens, principally from the Late Jurassic Solnhofen Formation, are small adults, transitional from larger ancestors to larger descendants. Tiny pterosaurs experiencing phylogenetic miniaturization(as in birds, mammals, crocs, turtles, basal reptiles, and many other clades) that helped their lineage survive while larger forms perished, Sadly, no tiny pterosaurs are known from the Late Cretaceous when they all became extinct.

References
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Codorniú L, Paulina-Carabajal A and Gianechini FA 2015. Braincase anatomy of Pterodaustro guinazui, pterodactyloid pterosaur from the Lower Cretaceous of Argentina. Journal of Vertebrate Paleontology, DOI:10.1080/02724634.2015.1031340

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